In this paper, a novel scheme for the estimation of the equivalent series resistance (ESR) of the dc-link electrolytic capacitor in three-phase ac/dc pulsewidth-modulation converters is proposed for condition monitoring. First, a controlled ac current component is injected into the input. Then, it induces ac voltage ripples on the dc output. By manipulating these ac voltage and current components with digital filters, the value of the ESR can be calculated, where the recursive least squares algorithm is used for reliable estimation results. In addition, the value of the ESR is corrected by considering the temperature effect, for which a simple temperature-sensing circuit has been designed. The simulation and experimental results show that the estimation error of the ESR is within a reasonable range, thereby enabling the determination of the appropriate time for the replacement of the capacitor.

Fault Diagnosis of DC-Link Capacitors in Three-Phase AC/DC PWM Converters by Online Estimation of Equivalent Series Resistance

Low voltage ride through of doubly-fed induction generator connected to the grid using sliding mode control strategy

In China, as in other parts of the world, many of the best resources for wind generation are located far away from load centers. Large generating facilities connected to distant load centers by long ac transmission lines face numerous technical challenges, regardless of the type of generating facility. This paper addresses some of the most significant challenges for wind generation facilities, including voltage control, reactive power management, dynamic power-swing stability, and behavior following disturbances in the power grid. Wind generation technology has evolved significantly over the past several years, and proven solutions to these technical challenges now exist. Controls integrated into the power electronics and mechanical controls of individual wind-turbine-generators, combined with integrated wind-farm control systems, have the capability of controlling numerous wind turbines so that they act as one unified generating plant at the point of interconnection with the power grid. This advanced hierarchical control of both real and reactive power output can provide dynamic performance that is, in many cases, superior to that achievable with modern conventional synchronous generation. This paper describes: ① Wind farm control functions, including performance for controlling grid voltage in quasi-steady-state and dynamic conditions. ② Low-voltage ride-through characteristics, including performance following severe system disturbances. ③ Dynamic power control functions within wind turbine-generators, including transient and dynamic performance for power swings.

Integrating Large Wind Farms Into Weak Power Grids With Long Transmission Lines

https://vbn.aau.dk/ws/files/289253331/Manuscript.pdf

Performance improvement of shunt active power filter based on non-linear least-square approach

A micro-grid islanding detection method includes the steps: installing a device node for implanting an improved active frequency drift method at a common connection point of a micro-grid and a main grid; setting threshold values of voltage and frequency, injecting disturbing current with the disturbance factor of K0 into a system in real time, detecting the voltage UPCC and the frequency f of the common connection point, and judging that islanding happens when the voltage UPCC or the frequency f exceeds the threshold values; performing next discrimination if the voltage UPCC or the frequency f does not exceed the threshold values, comparing the current frequency fin of the injected grid with grid voltage frequency fgrid under a normal condition by a comparison circuit, and determining load characteristics; calculating a disturbance factor K in positive feedback application according to the load characteristics to generate final disturbing signals; transmitting the disturbing signals to an inverter of a distributed power generation system by a PWM (pulse width modulation) driving module, disconnecting a grid-connected circuit breaker when the frequency and the voltage of the common connection point are changed to trigger over/under frequency and over/under voltage detection due to islanding, and stopping power supply for a load by the inverter until the grids return to normal

Micro-grid islanding detection method

The invention discloses a method for detecting a fundamental component and a harmonic component in voltage of a power grid, which relates to the field of signal processing and solves the problem that the traditional method for detecting the fundamental component and the harmonic component in the voltage has low accuracy. The method comprises the following steps of: designing a wave trap according to an LMS (Least Mean Square) algorithm; separating out the fundamental component and the sub-harmonic components from voltage of power grid by using the wave trap to obtain the fundamental component and the sub-harmonic components in the phase voltage; taking an error signal in the obtained fundamental component in the phase A voltage as a feedrear of a phase locked loop, and using the phase locked loop to obtain an estimation value of a power grid voltage frequency and an estimation value of the phase; and according to the obtained fundamental component and the sub-harmonic components in the phase voltage, using a symmetrical component method to detect positive sequence, negative sequence and zero sequence information of the fundamental component and the sub-harmonic components, taking the information as the detection results, and finishing the detection of the fundamental component and the harmonic component in the voltage of the power grid. The method is suitable for the detection of the fundamental component and the harmonic component in the voltage of the power grid.

Method for detecting fundamental component and harmonic component of voltage of power grid

Positive-negative sequence voltage feedforward method of three-phase PWM (pulse width modulation) converter

Island detection method of AFD (active frequency drift) based on synchronous rotation coordinate system

This paper presents an improved grid voltage control strategy for wind farms with doubly-fed induction generators (DFIGs) connected to distribution networks based on an analysis of the operation limits of DFIG systems. A modified reactive power limit calculation method in different operation states is proposed and a reactive power control strategy during grid voltage dips/rises is further discussed. A control strategy for compensating unbalanced grid voltage, based on DFIG systems, by injecting negative sequence current into the grid through the grid side converter (GSC) is proposed. In addition, the negative current limit of the GSC is discussed. The distribution principle of the negative sequence current among the different DFIG systems in a wind farm is also introduced. The validity of the proposed voltage control strategy is demonstrated by Matlab/Simulink simulations. It is shown that the stability of a wind farm and the power grid can be improved with the proposed strategy.

/pdf/improved-grid-voltage-control-strategy-for-wind-farms-with-1ltiye4w1e.pdf

Improved Grid Voltage Control Strategy for Wind Farms with DFIGs Connected to Distribution Networks

A robust synchronization method based on least mean square is proposed for the real time detection of fundamental and harmonic symmetrical components, including positive, negative and zero sequence. The new architecture is composed of three parallel structures providing estimation of each phase fundamental and harmonic components delivered into a linear matrix transformation for grid sequence extraction. To achieve frequency adaptation, a combination of least mean square and phase locked loop is used. The paper presents excellent simulation and experimental results in order to verify correctness and validity of the algorithm. Streszczenie. W artykule opisano opracowaną metode synchronizacji z siecią, wykrywającą w czasie rzeczywistym czestotliwośc podstawową oraz wyzsze harmoniczne, w tym skladową zgodną, przeciwną i zerową. Architektura sklada sie z trzech rownoleglych estymatorow harmonicznych dla kazdej fazy. Po przetworzeniu danych i wyznaczeniu skladowych, dokonywana jest synchronizacja, z wykorzystaniem metody najmniejszych kwadratow oraz PLL. Przedstawione zostaly wyniki symulacyjne i eksperymentalne. (Algorytm synchronizacji czestotliwościowej w ekstrakcji skladowych symetrycznych w sieci).

Yicheng Liu

Papers

Method for detecting fundamental component and harmonic component of voltage of power grid

Positive-negative sequence voltage feedforward method of three-phase PWM (pulse width modulation) converter

Island detection method of AFD (active frequency drift) based on synchronous rotation coordinate system

Improved Grid Voltage Control Strategy for Wind Farms with DFIGs Connected to Distribution Networks

A robust synchronization method for grid sequence extraction